Ethylbenzene is a major commodity chemical, most notably used as a feedstock in the preparation of styrene monomer. For many years, ethylbenzene was produced by employing Lewis acids to catalyze benzene ethylation, also referred to as the Friedels Craft reaction. Commercial processes used either boron trifluoride or aluminum chloride catalysts. During the 1960s zeolites with molecular size pores were introduced as acid catalysts for petroleum processing. Their use was extended to benzene ethylation as a way to eliminate the troublesome liquid-phase Lewis acid catalysts, which caused environmental and metallurgical difficulties.
Today, therefore, it is well known in the art to prepare ethylbenzene by alkylating benzene with ethylene in the presence of a suitable catalyst. Both single and multi-bed alkylation reactors are known. For example, in U.S. Pat. No. 3,478,119 (Maier et al.), substantially anhydrous benzene is alkylated with high purity ethylene over a solid phosphoric acid catalyst. U.S. Pat. Nos. 3,591,650 (Mitsak), 3,848,012 (Applegath et al.), and 3,766,290 (Carlson) describe processes for producing ethylbenzene by reacting ethylene with benzene in the presence of an aluminum chloride catalyst. U.S. Pat. No. 3,691,245 (Helzner) describes a process for producing ethylbenzene by alkylating benzene with ethylene, further including various process embellishments such as recovering benzene in the off-gases by recirculating a large portion of the polyethylbenzene byproduct, splitting the benzene-rich polyethylbenzene scrubber effluent, and processing both benzene-rich scrubber effluent together with the total reactor effluent to recover additional benzene, ethylbenzene and residual polyethylbenzene.
In U.S. Pat. Nos. 3,751,504 (Keown et al.), 3,751,506 (Burress), and 3,755,483 (Burress), the alkylation reaction is carried out in the vapor phase in the presence of a crystalline aluminosilicate zeolite characterized by a particular x-ray diffusion pattern. A number of more recent patents in this art, such as U.S. Pat. Nos. 4,169,111 (Wight), 5,073,653 (Buffer), 4,922,053 (Waguespack et al.), and 4,849,569 (Smith), describe variations on the zeolite-catalyzed alkylation process utilizing different zeolite catalysts and/or different process parameters. The foregoing prior art patents are incorporated herein by reference.
Conventionally, as taught by the prior art, ethylene is alkylated in the presence of a large excess of benzene (e.g. 6-9 moles bezene per mole of ethylene) over a zeolite catalyst. The excess benzene is distilled from the reactor effluent and recycled to the alkylation reactor. The aforementioned processes typically utilize polymer grade (about 99.8% pure) ethylene and "nitration" grade (about 99.7% pure) benzene. Cheaper supplies of ethylene, however, are available. One widely available and economically attractive source of ethylene for preparing ethylbenzene is a dilute ethylene feedstock, such as the offgas produced by fluid catalytic cracking units in petroleum refineries or by coke ovens. Such dilute ethylene feedstocks containing for example 10-25% ethylene by volume, however, are typically contaminated with a variety of both reactive and substantially inert components. The reactive impurities of greatest concern are propylene and heavier olefins, which are highly reactive with benzene during the alkylation reaction and result in the formation of heavier alkylbenzenes which then contaminate the ethylbenzene product stream. Furthermore, these unwanted reactions consume benzene and result in unwanted byproducts which must be subsequently separated and treated to recover the benzene or else burned with recovery of energy value.
To minimize the aforementioned problems, dilute ethylene feedstocks have previously been pretreated to reduce their propylene and heavier olefin contents from several percent to levels similar to those found in polymer grade ethylene prior to utilizing such feedstocks for ethylbenzene production. But, the prior art processes for pretreating dilute ethylene feedstocks to remove higher olefin contaminants are generally expensive and require two or more separate unit operations. For example, one relatively effective pretreatment process is to absorb the unwanted higher olefin contaminants in a naphtha solvent. For effectiveness and economy, however, this process also requires a downstream step to recover volatilized naphtha from the treated gas stream, as well as a downstream step to strip the absorbed olefins from the naphtha. Prior art refrigeration processes for pretreating dilute ethylene feedstocks are also generally effective but, for efficiency, also entail several additional operations and equipment completely unrelated to the ethylbenzene production process. These processing requirements have limited the utility of dilute ethylene feedstocks in the production of ethylbenzene and raised the costs of such operations. These and other problems with and limitations of the prior art are overcome with the dilute ethylene feedstock pretreatment process of the present invention.